As technology advances, there is a need for optical networks to become colorless, directionless, and contentionless (CDC). These types of networks require new colorless optical components that can re-route any optical signal from any input node to any output node.
An M×N multicast switch (MCS) is one of the components suitable for use in a CDC network. A typical M×N multicast switch 10 is illustrated in FIG. 1. As can be seen, the multicast switch 10 comprises a number M of 1×N splitters 121, . . . , 12M (collectively “splitters 12”) for splitting up optical signals received via M inputs to the switch 10. The multicast switch 10 also comprises a number N of M×1 switches 141, . . . , 14N (collectively “switches 14”) for collecting signals from the output of the splitters 12 and outputting the signals to selected ones of N outputs from the switch 10. A control mechanism (not shown) ensures that the switches 14 are set to output the correct signal.
It is known that PLC (planar lightwave circuit) technology is well suited for implementing the M×N multicast switch 10, as both the splitters 12 and switches 14 can be fabricated with good optical performance. However, and as can be appreciated, the difficulty arises in how to get all of the splitter 12 outputs connected to the appropriate inputs of all of the switches 14 without impacting the optical performance of the switch 10 and without making the switch 10 too large, complex or costly. The problem is compounded for larger switches 10 as the number of splitter 12 outputs and switch 14 inputs increases exponentially.
FIG. 1 illustrates the outputs of splitter 121 and 12M as being connected to certain inputs of the switches 14. As mentioned above, all of the splitter 12 outputs would need to be connected to appropriate inputs of all of the switches 14. Optical waveguides can be used for this purpose and are a viable solution to the connection problem described above. In one form, optical waveguides can be implemented as a mesh of light paths formed within the substrate containing the splitters 12 and switches 14. As can be seen in FIG. 1, there will be several points 13 (only a few of which are labeled) where the optical waveguides cross each other.
Unfortunately, each crossing adds an insertion loss (IL) to the throughput of the corresponding optical path, which is undesirable. It is known that a typical waveguide crossing loss is around 0.05 dB per crossing. As can be seen in FIG. 1, it is also apparent that the number of crossings will vary greatly: from none in the outer paths to (M−1)*(N−1) for some of the inner paths. This will lead to paths with higher insertion loss, which increases both the worst-case insertion loss as well as the insertion loss uniformity (ILU) of all of the paths in the switch 10.
Moreover, some of the light within one waveguide can be transferred to a crossing waveguide if the angle between the two waveguides is too low. As can be appreciated, the mesh gets more and more complicated as the number of splitters 12 and switches 14 increase, which leads to lower angles in some of the crossings. This situation is also undesirable because the overall isolation of the multicast switch 10 is degraded by the light transfer.
FIG. 2 illustrates an example 4×4 multicast switch 20 using a mesh 26 of optical waveguides to connect the outputs of splitters 22 to the inputs of switches 24. The outside paths such as path 231 has no crossings, while the other paths (e.g., path 232) can have can have up to nine crossings each. This means that the mesh 26 used in switch 20 experiences insertion loss anywhere from 0 to about 0.45 dB per optical path, which as noted above is undesirable.
FIG. 3 illustrates an example 8×8 multicast switch 30 using a mesh 36 of optical waveguides to connect the outputs of splitters 32 to the inputs of switches 34. The outside paths such as path 331 has no crossings, while the other paths (e.g., path 332) can have can have up to forty-nine crossings. This means that the mesh 36 used in switch 30 experiences insertion loss anywhere from 0 to about 2.45 dB per optical path. As can be seen, the FIG. 3 mesh 36 is much more complex than the FIG. 2 mesh 20, which leads to the greater insertion loss as well as low crossing angles (and potential crosstalk) in some of its paths.
There have been attempts to overcome similar problems in the past for other types of optical switches. For example, U.S. Pat. No. 4,787,692 discloses an optical switching element comprising multiple stages of active switches interconnected by optical waveguides. In one embodiment, the architecture of the '692 patent has four regions of crossovers, and other regions of switches. This arrangement, however, is still not suitable for today's needs.
U.S. Pat. No. 4,852,958 discloses an optical matrix switch comprising a tree-type structure having an input branching tree connected to an output merging tree through a stage of 2×2 switches, waveguides and dummy waveguides. The disclosed architecture is still complex, still has crossovers and requires active switches in addition to waveguide and dummy waveguide connections.
Jajszczyk et al., “Tree-type Photonic Switching Networks”, IEEE Network, vol. 9 no. 1 pp. 10-16 (1995), discloses various tree-type architectures for photonic switching networks, included guided-wave based switching elements. Each architecture has its advantages and disadvantages, uses different amounts of active elements, and experiences different types of crossovers, insertion loss, and signal-to-noise ratios. The disclosed architectures, however, do not reduce the number of crossings enough for today's technological requirements.
Thus, there remains a need to further reduce the number of waveguide crossings in optical switches such as M×N multicast switches and other M×N optical components.